Manufacture of precipitated silica



May22,1 934. s. s. SVENDSEN 1,959,747"

MANUFACTURE OF PRECIPITATED SILICA Filed Feb. 3. 1930 i rac-ebber(ncewed A IVI/SQ Il [III Patented May 22, 1934 PATENT OFFICE;

MANUFACTURE OF PRECIPITATED SILICA Svend S. Svendsen, Chicago, Ill.,assignorl to Clay 4Reduction Company, a corporation of IllinoisApplication February 3, 1930, Serial No. 425,671

24 Claims., (Cl. 23--l82) The silicon diammino tetrafluoride may beprepared by the reaction of van ammonium fluoride compound, such asnormal ammonium fluoride or ammonium bifluoride, upon a siliciousmaterial such as quartz, clay, talc or the like. ln my prior applicationSerial No. 191,267 filed May 13, 192,7, and copending Serial No.425,345, filed February l, 1930, I have fully described the manufactureof silicon diammino tetrafiuoride. The following briefly summarizes asatisfactory procedure. A suitable silicious material, which may bepreheated to facilitate'the reaction with the ammonium fluoridecompound, is reacted upon with the latter, either in solution or dry, ata temperature above 34 C. and, incase dryreaction is desired, above 100C;A` The reaction between the silica content of the mineral and theammonium fluoride compound proceeds apparently with the formation ofammoniumsilicofluoride, water and ammonia. Fluorides of metals presentin the silicious material are also formed. On further v heating,ammonium-silico-fluoride' is converted into silicon diamminotetrailuoride (SiF4 l(NH3) 2) and volatilizes at a temperature above 230C., and preferably above `250 C. For example, the reaction may becarried out as follows: l

About 120 tons of clay, preferably calcined at a red-heat to increaseits reactivity, containing 40 on analysis 25% alumina, 60% silica, 3%ferrous oxide, 2% lime, 4% soda and 6% potash, is mixed with an ammoniumfluoride solution containing 262 tons of ammoniurnfluoride. This issufficient toV convert the silica of the clay into silicon diam minotetrafiuoride and the metallic oxides into iluoridea The mixture isheated to between 34 and 100 C., preferably between 60 C. and 100 C. Theammonium fluoride is thereby dissociated into ammona and ammoniumbifluoride; vthe latter attacks the clay, .reforming normal 4ammcniumfluoride, which is again decomposed. Ammonia. gas is evolved andrecovered. Ammonium silicofluoride is formed. The`4 mixture isevaporated to dryness, and furtlerheated to above 230 C..` The ammoniumsilicofiuoride is thereby converted into silicon diammino tetrafluorideand metallic iluorides also are formed. At this temperature the silicondiammonio tetrailuoride is volatilized. The same ilnal result isachieved if dry ammonium fluoride crystals are 50 heated with the clay.

In the above example, if it is desired not-to leave a metallic fluorideresidue, only sufiicient 'ammonium fluoride is used with the clay toform silicon diammino tetrafiuoridev with the silicon present. Metallicfluorides for ed'y may be decomposed as described in my copendingapplications. i

As a further example of the production of silicon diamminotetrafluoride, quartz is heated 0 to a bright redheat, suddenly cooledby immersion in water and pulverized. While the pulverized quartz is ata temperature of about 15,0 to 200 C., ammonium-fluoride is addedgradually in a proportion suilicient "to 'convert the quartz 5 intosilicon diammino tetrauoride. Water and ammonia are evolved. Thetemperature is then raised to 230 to 300 C. and silicon diamminotetrafluoride is vaporized.

Silicon diammino tetrailuoride, formed for example as above described,may be condensed and collected, and then caused to react with water orwith water and ammonia. Preferably,I however, the silicon diammlnotetrafluoride is caused to react with water or with water and ammonia bycontacting it'therewith while in the vapor state, or during itscondensation from the vapor state. Previously condensed and collectedsilicon diammino' tetrauoride may be revaporized for reaction in thismanner, or the .vapors resulting from the reaction between the siliciousmaterial. and the vammonium fluorideV compound, which vapors containsilicon diammino ,tetrafluorida ammonia and water, may be employed inthe reaction.

When the silicon diammino tetrafiuoride is acted upon by water, ammoniumsilicon-fluoride,

ammonium fluoride and precipitated hydrated silica are formed. Thereaction is endothermic. When ammonia is added to this reaction mixture,the final products are ammonium-fluoride and precipitated hydratedsilica. This reaction is exothermic. When water and ammonia are bothpresent, the reaction proceeds directly to ammoniumfluoride andprecipitated silica as end products. l

The course of the reactions and the quantities of reagents required maybe vdetermined fromA the'following equations, wherein reference to thewater contained in the precipitated silica -is omitted.

The reaction expressed by Equation (2) is exothermic as is also thecombined reaction as expressed by Equation (3). The reactions should beconducted at a temperature below 34 C. for complete precipitation of thesilica, owing to the reversibility of the reaction expressed by Equation(2) above. Silica may be produced in the manner described attemperatures above 34 C., but the completeness of its precipitation isnot so great as at lower temperatures.

'I'he hydrated silica produced from the silicon diammino-tetrauoride bythe action of water (Equation l) and that precipitated by ammonia andwater from ammonium silicouoride (Equation 2) have varying and differentcharacteristics, as pointed out in my co-pending applications Serial No.543,027, filed June 8, 1931 and Serial No. 547,781, filed June 29, 1931.If the silicon' diammino tetrafluoride is subjected to the simultaneousreaction of water and ammonia, the resulting combination product tendsto have increased Whiteness and opacity, particularly if the reaction is,carried out in an ammonium fluoride solution in the presence ofammonia. The combination hydrated silica thus made in a strongammonium-fluoride solution is not voluminous and filters readily ascompared with the slower ltering products made in water or dilutesolutions. The opaque variety may, of course, be obtained separately 'bytreating silicon diammino tetrafluoride with water in the absence ofammonia, and filtering the silica so obtained from the solubleammonium-fluoride and ammonium- Silico-fluoride formed simultaneously(Equanon 1).

As hereinbefore stated; to effect the reaction, the condensed silicondiammino tetrafluoride may be added to water or to an ammoniacalsolution, for example, an ammonium hydroxide solution. However, it ispreferred to bring about the reaction by contacting silicon diamminotetrafluoride vapors with water, or with an ammoniacal aqueous liquid,or with water and gaseous ammonia, or combinations of these. AForexample, the silicon diammino tetraiiuoride vapors may be caused to passinto a spray tower into which water or ammonia solution or otherammoniacal aqueous liquid may be sprayed to simultaneously cool thesilicon diammino tetrafluoride vapors and condense the latter and bringabout the reaction re- -sulting in the formation of precipitated silica.

The liquid drawn olf as a result of this reaction contains theprecipitated silica in suspension and the precipitated silica may beremoved therefrom by any suitable means, such as filtration, and thefiltrate reused in the process. Or the filtrate may be drawn off and theammonium-fluoride content thereof recovered.,

When vapors ,derived from the reaction between silicious material. andammonium-uoride compounds are employed, the ammonia and water containedin such vapors are effective 'in replen- 4ishing the strength of thesolution employed for the reaction with the silicon diamminotetrauoride, or the ammoniagas may be used in the presence of water oraqueous solutions to effect the reaction with the silicon diamminotetrafluoride. I prefer, however, to employ an aqueous solution ofammonium uoride in the presence of ammonia, either gaseous or dissolvedtherein, for the reaction with the silicon diammino tetrauoride, as amore satisfactory, denser, more opaque, and more uniform precipitatedsilica is thereby secured, the opacity and density increasing with thestrength of the solution. When an aqueous solution of ammonium uoride inthe presence of ammonia is thus employed, the precipitated silica isfiltered therefrom, and a part of the filtrate is withdrawn so that theammonium fluoride therein may be employed in the reaction 'withsilicious material to produce silicon diamsolution, may be reused forthe reaction with silicon diammino tetrauoride, the ammonium'fluoridestrength of the solution being brought up or increased as a resultof thereaction.

For example, the operation of the present in- 1 vention may be carriedout in the following manner, as illustrated by the accompanying drawing,in which the figure represents diagrammatically, and partly in section,suitable apparatus for carrying the invention into effect. 1

As shown .in the drawing, the vapors, usually at a temperature of about'250 C. to 300 C., containing silicon diammino tetrafluoride, and alsocontaining ammonia and water vapor as derived from the reaction betweensilicious material and 1 ammonium fluoride, are introduced through a.pipe 5 into the lower portion of a spray tower 6. This tower may belead-lined, at least in its lower portion. The spray tower containspacking material 7, suitably broken ceramic material, Raschig 1 rings orthe like. The hot rising vapors are met by a stream of ammonium fluoridesolution containingmore or less dissolved ammonia, supplied in the upperportion of the tower through the spray 8. rlhe descending solutionreacts with the silicon diammino tetrafluoride vapors, the water andammonia dissolved therein taking part in the reaction and being in lpartor wholly replaced and aided by the water vapor'and ammonia brought inwith the silicon diammino tetrailuorde. The ammonium-fluoride formed inthe reaction increases the ammonium-uoride concentration of thesolution. The resulting liquid, containing suspended precipitatedsilica, is discharged from the tower 6 through the pipe 9, and collectedin a 130 tank 10. From this tank it is picked up by the suction line 11of pump 12, and discharged through the outlet line 13 to filter press14', in which the precipitated silica is removed from the ammoniumfluoride solution. The filtrate passes 1 out of the filter press 14through line 15 to a. receptacle 16. From the receptacle 16,ammoniumfluoride solution is drawn through line 17 by pumpl 18, whichdischarges it through line 19 into the line 20. From the line 20,thesolution maybe 1 passed through the cooling coil 21 to the line 22leading to the distributor 8 in the top of the tower 6. If desired, apart or all of the liquid may be bypassed around the coil 21 through.the line 23 provided with valve 24. The flow of liquid 1 through thecooling coil 21 may be controlled by valve 25. By controlling thecooling of the liquid entering the tower 6, the desired` reactiontemperature, `prefere/bly below 34 C., is maintained therein. 15o

' vantages.

In` amodification of the process, the solution leaving the tower ispassed through the coil 21 and is cooled to below 34 C. before passingthrough filter press 14 to ensure complete reaction as shown in Equation3, and coil 2l may be by-passed. In this method of operation the liquidin the tower reaches a temperature considerably above 34 C. beforepassing out at the bottom through pipe 9, due to the exothermiccharacter of the reaction and to the large amount of heat which isabsorbed from the entering .gases,`which usually are at about 250 to 300C. This higher operating temperature has some ad- The ammonia gas has amuch-lower solubility in the hot ammonium fluoride liquor so that theconcentration of ammonium-fluoride may become high in the liquor. Byapplying a vacuum, the ammonia concentration can be further reduced.This is desirable for reasons which will become apparent hereinafter.-The excess ammonia gas is vented from tower 6 through ppe 32 to asuitable absorbing orrliquefying apparatus. If the liquid leaving tower6 is considerably above 34 C., the heat contained therein may also beused advantageously, as by means of cooling coil 2l to heat th'e coldammonia saturated ammonium-fluoride solution. from which ammoniumfluoride has been crystallzed as describedl hereinafter.` Heating .thisliquid drives the ammonia therefrom to allow it to be used again infurther crystallizing operations, the lquid then being returned to tower6 to react with further mixed .silicon diammino tetrafluoride andammonia vapors.

The ammonium-fluoride concentration of the liquid employed in the tower6v may be varied -wi'thin wide limits, say from V10 t`o 50%.l .How-ievenl prefer to employ a solution of from 20 to 40% ammoniumeiluoridecontent and in general near 40%. The precipitated silica, which isnearly colloidal in ilneness, stays in suspension in this circulatingliquid and therefore need not be removed therefrom, as it does notysettle out to, clog pipes or other apparatus, provided the liquor isnot entirely quiescent. It needs to be filtered oi only when theconcentration of ammonium-fluoride is high enough, preferably about 40%or higher, to be removed by crystallization as described hereinafter;

In starting operations, for example, a solution of about 20%ammonium-fluoride content may be employed. It is recirculated in theinitial stages'of operation until its ammonum-fluoride concentration isbuilt up to 35 to 40%. lTo facilitate this circulation, a recirculatingline 26, controlled by valve 27 is provided from the outlet line 13 ofpump l2 to the line 20. A valve 28 is provided in the line 13 to preventflow of liquid 4through the' lter press 14, duriig this stage of theoperation. Any part or all` of the ammonium-fluoride thus recirculatedmay be passed through the cooling coil 21to maintain a temperature intheupperportion of the tower 6 of 34 C. or lower. Upon the ammonium-fluor-Aide solution reaching the desired concentration,i the valve 27 is closedto shutoff the reclrculating Y line 26, and the valve 28 is opened. 'Iheammonium-uoride solution containing the preciptated silica is thenforced through the filter press 14 and the filtrate is collected in thereceiver 16. From the latter it is withdrawn through the line 1'1 bythepump 18. and discharged through the line 19 to the 1ine.20, from whichit passes through the cooling coil 21 back to the line 22 and into thetower 6 through'. the distributor 8T The, proportion ofammonium-fluoride recirculated in this way' is controlled 4by the valve29.

in the line 19 and by the operation of the pump v18. Additional watermay be supplied to the ammonium-fluoride solution entering the tower 6through the valved line 30. Excess ammoniumiiuoride solution may bedischarged from the. receiver 16 through the discharge line 31. maloperation, the discharge of ammonium-fluoride solut'on and the supply ofadditional water' .using the counter-current principle, the resultingwash water being used in the absorption system. The filter cake is driedat a temperature suitable' for its use. By drying at above 230 C.

to 250 C. the last traces of ammonium-fluoride are removedA by thevolatilization of` that 'compound as silicon diammino tetrailuoride.-'I'he dried lter cake is preferably sifted throughv a iiour bolter todisintegrate the friablen lumps which may be formed. f

The precipitated silica obtained by reaction of water or of -water andammonia on silicon diammino tetrafluoride and especially on vaporizedsilicon diammino tetrauoride as hereinbefore described, is a distinctproduct having physical properties differing widely from those ofhydrated silica as obtained in the ordinary way. It does notform a gelwhen precipitated, but forms an easily filtered white suspension, andafter iiltering and washing, yields a white,

clay-like product of high plasticity with an apordinary silica gelagglomerates. It dries to an` opaque, white impalpable powder of lowapparent density having a clay-like feel, the dry .material havingapproximately the same volume as the washedlter cake before drying/Theparticles o'f the precipitated silica are extremely small and vary.somewhat in size with the mode of precipitation and treatment.remarkable uniformity in size, `being about onehalf to two microns ind`iameter,the air dried material being somewhat larger thangthecallcined material. Most of the air dried material averages. about' vone'micron' in diameter. For any one condition of precipitation andtreatment the particle size does not vary more than a fraction of amicron, in contrast to the widely varying sizes of the individualparticles of finely gound silica. `The individual particles are round tooval in shape when viewed under the microscope.

The air dried precipitated silica after screening or bolting weighsvfrom l2 to '15 pounds cubic foot. The air dried or moderately heatedindividual particles have the properties of a` per'y does not settleappreciably` even afterstanding .E

several months. This is in contrast to the rapid settlingycharacteristic of-finely groundl silica now used as an extender inpaints. `Ithasa a slight abrasive action and may be polishing powder. v

The air dried precipitated silica contains about 12% to 15% of water,varying with the humidity ofthe air. Itgmay be less in extremely dry airand somewhat higher than this range invery mama air. Dried at 11b o., itretains about l5% of watenbut on cooling and exposure to air, adsorbswater and reverts to about the same Water content as the air driedmaterial. Dried at higher temperatures, it still retains some wateradsorbing power, but to a less degree. When calcined at 800 C., it losesits entire water content and the calcined product has relatively slightwater adsorbing action.

The ammonium-fluoride filtiate may be used as a raw material for theproduction of fluoride salts, such as cryolite, or ltheammonium-fluoride may be crystallized out and reused in the reaction on'silicous material for the production of silicon diammino tetrafiuoride.However, if the ammonium-fluoride solution is evaporated to Yeffectcrystallization, it gives` off ammonia and hydrofluo'ric acid and theextremely corrosive nature of the latter makes it; very difficult? tofind vsuitable material for conducting the evaporation step. I havefound that by dissolving ammonia iiuoridel shows little difference insolubility in cold and warm water, it is desirable to chill orrefrigerate the solution when ammonia is introduced into it to securethe greatest yield of am# monium-nuoride crystals. If, for instance, to

'I .40%ammonium-fluoridesolution at 0 C. half of vits weight of dryammonial is added, about oue-` half ofthe ammonium-fluoride presentcrystal- `lizesl out. Substantially dry ammonia gas or -liquid ammonialmay be used; for example, am-

monia given off during the reaction between the silicious material andthe ammonium-fluoride compound may be employed for this purpose. A

lconsiderable amount of heat is generated andto improve thecrystallization, the liquid, as DIC- viously explained, should becooled', preferably to about' 0 C. or. lower, during the crystallizationprocess. After filtering out the ammoniumfluoride crystals and removingammonia from the residual uoride solution by heating, for example,

passing around cooling coil 21, and/or by applica..

tion of vacuum, the latter may be returnedl to the Although Ihave'described in detail a'method absorption tower system.

for treating the freshly formed gaseous Asilicon diammino tetrafiuoridewith coldammoniacal liquor to decompose it, the decomposition also pmay-be effected with solid silicon diammino .tetraiiuorida This compoundmay be condensed as a white fluffy crystalline solid which is stable inthe air at room temperature. The solid com-` pound also may bedecomposed with ammonia water at below 34 C. to form precipitated,vr'hydrate'ad silicagand ammonium fluoride.,

As shown in reaction No. 2, ammonium-silicofiuoride reacts with ammoniaand water to ,form precipitated silica. and ammonium fluoride. 'Ammoniumsilicofluoride is also a volatile compound and may be formed undercertain conditions of reaction when ammonium fluoride is heatedwith asilicious compound, especially when the-am monium fluoride is in excess.This compound may be volatilized with the silicon diammino tetrafluoridewithout impairing the formation of precipitated silica when the vaporsreact with the ammonia and water in the absorption tower.

Silicon` diammino tetraiiuoride may also be made by reacting on siliconiiuoride; with ammonia,

Titanium ammino tetrafiuoride'may be made from titanium compounds in thesame way that silicon diammino tetrafluoride may be made and issimilarly decomposed to finely divided titanium oxide by the action ofammonia and water. Whenl titanium is present in a'silicious compoundwhich is subjected to the action of ammonium fluoride at a temperatureof about 300 C., the titanium and silicon ammino. tetrauoride 'compoundsvolatilize off together. It is to be noted that titanium and silicon areadjacent members in Group IV. of the Periodic table. Vanadium compoundsreact in the same manner, forming fluoride-ammonia compounds which aresimilarly decomposed. to form oxides.

Although the present invention .has been described -in connection withspecific."de tails of operations in accordance therewith', it is notintended that these details shall be regarded as limitations upon thescope of` the invention, ex cept in so far as included in theaccompanying claims. -I'claim:.

1.*The method of forming precipitated oxides of metals ofthe groupconsisting of silicon, vanadium .and titanium which comprises treatinglthe amx'nino tetrafluoride compounds of ,metals 4of said group with anaqueoustsolution having an initial appreciable content of ammoniumfluoride.

2. The method of formingl precipitated oxides of metals of the groupconsisting of silicon, vana.- dium and titanium'which comprisescontacting vvaporsfof the ammino tetrafluoride compounds of said metalswith water, cooling the vapors and causing reaction between them and thewater.y

3. The method of producing precipitatedsilica which comprises contactingsilicon diammino -tetrafluoride with'an aqueous solution having aninitial appreciable content o f ammonium-fluoride.

4. The method of producing precipitated silica, which comprisescontacting silicon diamminotetrafiuoride vapors with an aqueous solutioncontaining an initial appreciable content of am' monium-fluoride in thepresence of ammonia.

5. The method of producing precipitated silica -tetrafiuoridevapors withan aqueous solution ence of ammonia. v

Y. i The method of producing precipitatedsilica whichcomprisescontacting.`a mixture of am.

of at least 30% ammonium-fluoride in the presmonia and silicondiamminotetrauoride vaporsmonia,'thereby forming precipitatedsillca, re-

which comprises contactingl silicondiammino discharging a portion of thesolution from the system, replenishing the volume thereof with water andreturning the solution for further treatment with silicon diamminotetrafluoride.

9. The method of producing precipitated silica, which comprisescontacting silicon diammino tetrailuoridel with an aqueous solution ofammonium-fiuoride in the presence of ammonia, thereby formingprecipitated silica in said solution, removing 'precipitated silica fromsaid solution, and returning such ammonium-fluoride solution to becontacted with additional silicon diammino tetrauoride. I

1Q. The Lmethod of producing precipitated silica which comprisescontacting a mixture of ammonia and silicon diammino tetraiiuoridevapors with water, thereby forming precipitated silica, removingprecipitated silica from the liquid, and again contacting the liquidwithadditional mixed vapors of ammonia and silicon diammino tetrauoride.

11. The method of producing precipitated silica which comprisescontacting silicon diammino tetrafluoride vapors with an aqueoussolution of ammonium-iiuoride in the presence oi ammonia,

thereby forming precipitated silica and additional ammonium-fluoride insaid solution, removing precipitated silica from said solution, andreturning ammonium-fluoride solutionwith additional water to be againcontacted with silicon diammino tetrafluoride vapors.,

12. The method of producing precipitated sil' ica which comprisescontacting silicon diammino tetrafluoride with an aqueous solution ofammomum-fluoride in the presence of ammonia, thereby formingprecipitated silica in said solution, cooling said solution to completethe reaction, removing precipitated silica-from said cooled solu- .tion.and returning such ammonium-humide which comprises contacting silicondiammino tetrafluoride withaqueous ammonium-fluoride solution in-thepresence of ammonia, 'theneby `forming precipitated silica andadditional am-` monium-fluoride, returning said solution and contactingit further with silicon diammino tetrafluoride until a desiredconcentration of ammonium-fluoride is secured in the solution.

15. Inthe method of producing precipitated `silica wherein silicondiammino tetrauoride vapors are contacted with aqueous ammoniumiiuorideHsolution in the presence of ammonia, thereby forming precipitated silicaand additional ammonium-fluoride, the combination of steps .fluoridevapors.

comprising returning said solution for further contact with silicondiammino tetrafiuoride vapors to secure a desired concentration ofammonium-nuoride therein, maintaining a constant recirculation of amonium-luoride solution of the desired concentration in contact with thesilicon diammino tetrafluoride `vapors, withdrawing a portion of theammonium-fluoride solution from said circulation and supplyingadditional water thereto, thereby maintaining theI desired concentrationof ammonium-fluoride.

16.'In the method of producing precipitated silica, contacting silicondiammino tetrauoride vapors' with ammonium-humide solution in thepresence of ammonia, whereby precipitated silica is formed and theammonium-fluoride concenmonium-iiuoride solution and precipitatedsilica,

yseparating the precipitated silica-therefrom, supplying ammonia in theammonium-fluoride solution, thereby crystallizing `a portion of theammonium-iiuoride therefrom, and returning the inother liquor containingammonium-fluoridev to be again contacted with silicon diammino tetra-1'7. The method of producing a mixture of precipitated silica andtitanium oxidefwhich comprises treating a mixture oi'titanium amminotetrauoride and silicon diammino tetrafluoride with water in thepresence ofl ammonia.

13.l The method ofproducing a mixture of pre- `cipitated silica andtitanium oxide which comprises treating mixed vapors of titanium amminotetrafiuoride and silicon diammino tetrauoride y with water, cooling thevapors and causing reaction 'to form the mixed oxides. 4

19.The method ofproducing a mixture of precipitated silica and titaniumoxide which comprisesgtreating mixed vapors of titanium amminotetrafluoride and silicon diammino tetraili'ioridev with water in thepresence of ammonia, cooling the vapors and causing reaction to form themixed oxides. l

20. The method of producing precipitated oxides of the metals of thegroup consisting of silicon, vanadiumcnd titanium which comprisescontacting ammino tetrailuoride compounds of a metal of said groupwithwater in the presence of ammonia, thereby forming precipitated oxide ofthe metal, removing the precipitated oxide from the liquid, and againcontacting the liquid with additional ammino tetrafluoride compounds ofthe metal in the presencesof ammonia.

21. The method of producing precipitated oxides of the metals of thegroup consisting of silicon, vanadium and titanium which comprisescontacting ammino tetraiiuoride compounds of a metalof said group withwater in the presence ofammonia, thereby forming precipitated oxides yofthe metal in said solution, removing tie oxides from the solution,discharging a portion of the solution from the system, replenishing thevolume thereof with water, andreturning the slution for furthertreatment with the ammino tetrafluoride compound.

22. The method of producing precipitated oxides of metals ofthe groupconsisting of silicon, titanium and vanadium which comprises contactingammino tetrafluoride compounds of such metals with water in the presenceof ammonia, thereby forming precipitated silica, and recirculating theliquid to contact with further quantities of said ammino tetrauoridecompound in the presence of ammonia.

23. The method of producing precipitated silica. which comprisescontactingy siiicon-diammino tetrafiuoride yapors withvan aqueoussolution of ammonium fluoride in the presence ot ammonia.

thereby cooling said vapors and causing reaction to formprecipitatedsilica.

SVND s. svENDsN.

VIVmi

